Heterogeneity and Microstructural Features
Intervening in the Ductile-Brittle Transition of
Ferrite-Pearlite Steels
Octobe...
Introduction
• Mechanical strength is properly described by mean
grain sizes in ferrite-pearlite structures.
• Toughness p...
Objectives
– Analysis of the behavior of several ferritepearlite microstructures with different local
heterogeneity.
• Gra...
Steel composition and Techniques

EXPERIMENTAL
Material and Heat Treatments
• CMn steel
C
0.1

Heat treatment #
1
2
3
4

Mn
0.48

Si
Al
N
0.006 0.041 48 ppm

Thermal cyc...
Experimental Procedure
• Optical Microscopy
• Philips XL30CP Scanning Electron Microscope (SEM). TSL
(TexSEM laboratories)...
Austenite and Transformed Structures

MICROSTRUCTURAL
CHARACTERIZATION
Austenite Grain Sizes
HT 2: 910ºC + 1.5ºC/s

HT 3: 980ºC + 0.1ºC/s

HT 4: 1000ºC + 0.1ºC/s

• HT #2: fine and homogeneous ...
Austenite Grain Sizes
HT 2: 910ºC + 1.5ºC/s

Area Fraction

0.3

HT 3: 980ºC + 0.1ºC/s

0.2

0.1

HT 4: 1000ºC + 0.1ºC/s

...
Transformed Microstructures
Sample 1: As-wrought

Dα = 28.3 µm
HT 3: 980ºC + 0.1ºC/s

Dα = 25.4 µm

HT 2: 910ºC + 1.5ºC/s
...
Transformed Microstructures
Accumulated Area Fraction

1

0.8

0.6

0.4
Treatment 1
Treatment 2

0.2

Treatment 3
Treatmen...
Mechanical Properties

CHARPY TESTS
Charpy Tests
400
Sample 1: As-wrought
(a)

350

Treatment #1

300

Absorbed Energy (J)

Absorbed Energy (J)

350

400

250...
Fractography
HT #3: Test @ -20ºC

No inclusions detected
in the origin
Fractography
HT #4: Test @ -40ºC
Fracture Initiation Ductile-Brittle Transition
#1: Test @ 27ºC

#4: Test @ -7ºC

• Energy absorbed by plastic deformation ...
Fractography
Etched Fracture Surface:
Grain boundary Carbides revealed as initiators
GB
carbides
Treatment

Pearlite

Grai...
Facet Size Distribution Measurements
Sample 1: As-wrought
0.7

(a)

Frequency

0.6

Treatment #1

0.5

Facets

0.4

Grains...
Facet Size Distribution Measurements
Sample 1: As-wrought

HT 2: 910ºC + 1.5ºC/s

0.7

0.7

(a)
Treatment #1

0.5

Facets
...
Microstructural Characterization by EBSD
Sample 1: As-wrougth

—2º~12º
— >12º
Microstructural Characterization by EBSD
HT 2: 910ºC + 1.5ºC/s

—2º~12º
— >12º
Crystallographic Measurements by EBSD
Treatment

Mean ferrite
size OM (mm)

Low angle
boundary fraction
(<12º)

1

28.3

8...
50% ITT

DUCTILE BRITTLE
TEMPERATURE PREDICTION
Ductile-Brittle Temperature Prediction



50%ITT  19  44%Si  700 % N f  2.2% pearlite  11.5 Dmean

80

Predicted 5...
Ductile-Brittle Temperature Prediction
50%ITT  87  44%Si  700 % N f  2.2% pearlite  11.5Dc20%

0.5

80

Predicted...
Extension to Nb-Mo Microalloyed Steels.
Ductile-Brittle Temperature Prediction

Predicted 50%ITT (ºC)

50

-50

3NbMo0

-1...
Extension to Nb-Mo Microalloyed Steels.
Ductile-Brittle Temperature Prediction

Predicted 50%ITT (ºC)

50

-50

3NbMo0

-1...
Final Remarks

CONCLUSIONS
Final Remarks
• Toughness of ferrite-pearlite microstructures:
– importance of microstructural heterogeneity.
– contributi...
Acknowledgements

• Financial support by:
– Spanish Ministry of Economy and
Competitiveness (MAT2009-09250)
– Basque Gover...
Heterogeneity and Microstructural Features
Intervening in the Ductile-Brittle Transition of
Ferrite-Pearlite Steels
Octobe...
Extension to Nb-Mo Microalloyed Steels.
Ductile-Brittle Temperature Prediction

Predicted 50%ITT (ºC)

50

-50

3NbMo0

-1...
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Heterogeneity and Microstructural Features Intervening in the Ductile-Brittle Transition of Ferrite-Pearlite Steels

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In conventional heat treatments or rolling schedules, the microstructure can be properly identified by its mean attributes. These parameters can properly predict the ductile-brittle (DB) transition temperatures measured by Charpy tests. However, if the austenite grain size distribution prior to transformation remains heterogeneous, after transformation, wider distributions of grain sizes will be obtained. In this context, the classical approaches do not properly predict the DB regime. This study analyzes the behavior of several ferrite-pearlite microstructures with different local heterogeneities. Grain size distributions, EBSD analysis identifying high angle misorientation boundaries and cleavage facet measurements were performed. These parameters have been incorporated in previous empirical expressions in order to quantify the contribution of these heterogeneities to the DB transition temperature.

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Heterogeneity and Microstructural Features Intervening in the Ductile-Brittle Transition of Ferrite-Pearlite Steels

  1. 1. Heterogeneity and Microstructural Features Intervening in the Ductile-Brittle Transition of Ferrite-Pearlite Steels October 29, 2013 – Montreal, Quebec Canada R. Zubialde, P. Uranga, B. López and J.M. Rodriguez-Ibabe puranga@ceit.es (CEIT and TECNUN, Univ. Navarra) San Sebastian, Basque Country, Spain
  2. 2. Introduction • Mechanical strength is properly described by mean grain sizes in ferrite-pearlite structures. • Toughness prediction is not straightforward with average grain sizes. • Classical equations include dα and %pearlite to predict the ductile-brittle (DB) transition temperatures. • However, if austenite distribution is not properly controlled – Austenite heterogeneity → heterogeneous ferrite distributions. – Weakest link behavior: Coarsest grains will trigger brittle fracture.
  3. 3. Objectives – Analysis of the behavior of several ferritepearlite microstructures with different local heterogeneity. • Grain size distributions, EBSD analysis identifying low/high angle misorientation boundaries and cleavage facet measurements. – Incorporation in previous empirical expressions to quantify the contribution of the heterogeneity to the ductile-brittle (DB) transition temperature.
  4. 4. Steel composition and Techniques EXPERIMENTAL
  5. 5. Material and Heat Treatments • CMn steel C 0.1 Heat treatment # 1 2 3 4 Mn 0.48 Si Al N 0.006 0.041 48 ppm Thermal cycle As-wrought microstructure 910ºC for 30 minutes and air cooling at 1.5ºC/s 980ºC for 30 minutes and furnace cooling at 0.1ºC/s 1000ºC for 30 minutes and furnace cooling at 0.1ºC/s
  6. 6. Experimental Procedure • Optical Microscopy • Philips XL30CP Scanning Electron Microscope (SEM). TSL (TexSEM laboratories) MSC 2002 equipment. • Field Emission Scanning Electron Microscope (FEG-SEM) Jeol JSM-7000F. HKL Channel5 EBSD • Charpy tests
  7. 7. Austenite and Transformed Structures MICROSTRUCTURAL CHARACTERIZATION
  8. 8. Austenite Grain Sizes HT 2: 910ºC + 1.5ºC/s HT 3: 980ºC + 0.1ºC/s HT 4: 1000ºC + 0.1ºC/s • HT #2: fine and homogeneous austenite (Dγ = 15 μm) • HT #3 and 4: heterogeneous austenite (Dγ = 37 and 25 μm). − HT #3: coarse austenite grains (400 μm approx.) within a fine matrix. − HT#4: coarse austenite structure (200 μm approx.) with fine austenite grains decorating the grain boundaries.
  9. 9. Austenite Grain Sizes HT 2: 910ºC + 1.5ºC/s Area Fraction 0.3 HT 3: 980ºC + 0.1ºC/s 0.2 0.1 HT 4: 1000ºC + 0.1ºC/s 0 5 0.3 35 50 65 80 0.2 0.3 Area Fraction Area Fraction 20 Austenite Grain Size (mm) 0.1 0.2 0.1 0 0 20 80 140 200 260 320 380 Austenite Grain Size (mm) 20 80 140 200 260 320 380 Austenite Grain Size (mm)
  10. 10. Transformed Microstructures Sample 1: As-wrought Dα = 28.3 µm HT 3: 980ºC + 0.1ºC/s Dα = 25.4 µm HT 2: 910ºC + 1.5ºC/s Dα = 10.3 µm HT 4: 1000ºC + 0.1ºC/s Dα = 30.6 µm
  11. 11. Transformed Microstructures Accumulated Area Fraction 1 0.8 0.6 0.4 Treatment 1 Treatment 2 0.2 Treatment 3 Treatment 4 0 0 50 100 Ferrite Grain Size (mm) Treatment # 1 2 3 4 Proeutectoid ferrite fraction % 88 90 89 88 Ferrite mean size (µm) 28.3 10.3 25.4 30.6 150
  12. 12. Mechanical Properties CHARPY TESTS
  13. 13. Charpy Tests 400 Sample 1: As-wrought (a) 350 Treatment #1 300 Absorbed Energy (J) Absorbed Energy (J) 350 400 250 200 150 100 300 250 200 150 100 50 0 HT 2: 910ºC + 1.5ºC/s (b) 50 0 Treatment #2 -80 -60 -40 -20 0 20 40 -80 60 -60 -40 HT 3: 980ºC +400 0.1ºC/s 0 20 40 60 400 350 300 300 Absorbed Energy (J) 350 Absorbed Energy (J) -20 Temperature (ºC) Temperature (ºC) 250 200 150 100 50 (c) Treatment #3 0 250 200 150 100 HT 4: 1000ºC + 0.1ºC/s (d) 50 Treatment #4 0 -80 -60 -40 -20 0 20 40 60 -80 -60 Temperature (ºC) Treatment # 1 2 3 4 50% ITT (ºC) 28 -30 -4.9 -6.5 -40 -20 0 20 Temperature (ºC) 27J (ºC) 12 -39 -8 -15 54 J (ºC) 18 -36 -7 -12 40 60
  14. 14. Fractography HT #3: Test @ -20ºC No inclusions detected in the origin
  15. 15. Fractography HT #4: Test @ -40ºC
  16. 16. Fracture Initiation Ductile-Brittle Transition #1: Test @ 27ºC #4: Test @ -7ºC • Energy absorbed by plastic deformation until brittle fracture happens. • Brittle fracture initiation areas isolated by a ductile region. • Crack energy lower than the matrix/matrix interface energy. • First facet size 2-3 times bigger than average grain size.
  17. 17. Fractography Etched Fracture Surface: Grain boundary Carbides revealed as initiators GB carbides Treatment Pearlite Grain Boundary Cementite Thickness (mm) 1 0.6 2 0.5 3 0.54 4 0.55
  18. 18. Facet Size Distribution Measurements Sample 1: As-wrought 0.7 (a) Frequency 0.6 Treatment #1 0.5 Facets 0.4 Grains 0.3 0.2 0.1 0 10 30 50 70 90 110 130 150 170 Size (mm) Ni Secondary Crack stopped at a grain boundary
  19. 19. Facet Size Distribution Measurements Sample 1: As-wrought HT 2: 910ºC + 1.5ºC/s 0.7 0.7 (a) Treatment #1 0.5 Facets 0.4 Grains 0.3 0.2 0.1 (b) 0.6 Frequency Frequency 0.6 Treatment #2 0.5 Facets 0.4 Grains 0.3 0.2 0.1 0 0 10 30 50 70 90 110 130 150 170 10 30 50 Size (mm) 90 110 130 150 170 Size (mm) HT 4: 1000ºC + 0.1ºC/s HT 3: 980ºC + 0.1ºC/s 0.7 0.7 (c) 0.6 Treatment #3 0.5 Facets 0.4 Grains 0.3 0.2 (d) 0.6 Frequency Frequency 70 Treatment #4 0.5 Facets 0.4 Grains 0.3 0.2 0.1 0.1 0 0 10 30 50 70 90 Size (mm) 110 130 150 170 10 30 50 70 90 Size (mm) 110 130 150 170
  20. 20. Microstructural Characterization by EBSD Sample 1: As-wrougth —2º~12º — >12º
  21. 21. Microstructural Characterization by EBSD HT 2: 910ºC + 1.5ºC/s —2º~12º — >12º
  22. 22. Crystallographic Measurements by EBSD Treatment Mean ferrite size OM (mm) Low angle boundary fraction (<12º) 1 28.3 8.4% 21.0 26.1 77 2 10.3 7.5% 10.7 11.9 30 3 25.4 9.4% 22.0 22.3 57 4 30.6 9.4% 25.0 25.1 75 5º mean 12º mean size (mm) size (mm) Dc20% (mm) 90 80 70 0.8 Dc20% (mm) Accumulated Area Fraction 1 0.6 0.4 Treatment 1 60 50 40 30 Treatment 2 0.2 20 Treatment 3 Treatment 4 0 50 100 Ferrite Grain Size (mm) 𝐷𝑐~3𝐷 𝑚𝑒𝑎𝑛 10 0 150 0 0 10 20 Ferrite 12º Grain Size (mm) 30
  23. 23. 50% ITT DUCTILE BRITTLE TEMPERATURE PREDICTION
  24. 24. Ductile-Brittle Temperature Prediction   50%ITT  19  44%Si  700 % N f  2.2% pearlite  11.5 Dmean 80 Predicted 50%ITT (ºC) 60 40 20 0 -20 -40 Equation 1 -60 -60 -40 -20 0 20 40 Experimental 50%ITT (ºC) 60 80  0.5  112t 0.5
  25. 25. Ductile-Brittle Temperature Prediction 50%ITT  87  44%Si  700 % N f  2.2% pearlite  11.5Dc20% 0.5 80 Predicted 50%ITT (ºC) 60 40 20 0 -20 -40 Equation 2 -60 -60 -40 -20 0 20 40 Experimental 50%ITT (ºC) 60 80  112t 0.5
  26. 26. Extension to Nb-Mo Microalloyed Steels. Ductile-Brittle Temperature Prediction Predicted 50%ITT (ºC) 50 -50 3NbMo0 -150 3NbMo31 6NbMo0 6NbMo31 -250 -250 -150 -50 Experimental 50%ITT (ºC) 50 50%ITT(ºC)  Composition  Secondary Phases(%pearl  %M/A  D M/A )   Precipitation  Dmean  Dc20%
  27. 27. Extension to Nb-Mo Microalloyed Steels. Ductile-Brittle Temperature Prediction Predicted 50%ITT (ºC) 50 -50 3NbMo0 -150 3NbMo31 6NbMo0 6NbMo31 CMn -250 -250 -150 -50 Experimental 50%ITT (ºC) 50 50%ITT(ºC)  Composition  Secondary Phases(%pearl  %M/A  D M/A )   Precipitation  Dmean  Dc20%
  28. 28. Final Remarks CONCLUSIONS
  29. 29. Final Remarks • Toughness of ferrite-pearlite microstructures: – importance of microstructural heterogeneity. – contribution of the largest grains in the toughness of the material is one of the key factors controlling brittle behavior. – a modified equation has been proposed to accurately predict ductile-brittle transition temperature. • Strategy extension to microalloyed steels with complex microstructures
  30. 30. Acknowledgements • Financial support by: – Spanish Ministry of Economy and Competitiveness (MAT2009-09250) – Basque Government (PI2011-17)
  31. 31. Heterogeneity and Microstructural Features Intervening in the Ductile-Brittle Transition of Ferrite-Pearlite Steels October 29, 2013 – Montreal, Quebec Canada R. Zubialde, P. Uranga, B. López and J.M. Rodriguez-Ibabe puranga@ceit.es (CEIT and TECNUN, Univ. Navarra) San Sebastian, Basque Country, Spain
  32. 32. Extension to Nb-Mo Microalloyed Steels. Ductile-Brittle Temperature Prediction Predicted 50%ITT (ºC) 50 -50 3NbMo0 -150 3NbMo31 6NbMo0 6NbMo31 CMn -250 -250 -150 -50 Experimental 50%ITT (ºC) 0.5 50%ITT(ºC)  11M n  42Si  700(N free )  0.5Δ y  14(D 50  15(%pearl  %M /A)1/3  ) -0.5  1.4( Dc20% )1.5  23.9(D M/A ) 0.5 mean_15º D mean_15º

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